Since that layer is preserved world wide the chances of finding a fossil of something covered by that layer should be pretty good. Since I wrote the question it has come to me. Acid rain. The rock blasted by the impact had a high sulfur content. That sulfur came back down as acid rain and that must have dissolved all the dead things that would have been either under or on top of that layer. Either that or the dinosaurs were already mostly gone before the impact. And that the very few that died off just over the layer were in small enough numbers that there just weren't anything left to fossilize. Am I getting warm?